The baking of bread, as in many other cases involving convective/conductive heat transfer, is very wasteful of heat energy. In order to bake the bread, it is necessary to first heat the walls of the oven and the air within the oven so that this heat energy may be transferred to the surface of the bread and then slowly carried to the interior via conductive heat transfer. The process is largely limited by the rate at which heat can be conducted from the surface of the bread to the interior. While the process can be speeded up somewhat in a conventional oven through the use of higher ambient temperatures, this is rapidly limited by the temperature at which the surface begins to brown excessively and bakeout becomes too high, as the major negative effects. The result of this is that white bread is conventionally baked for 17 to 23 minutes at 400.degree. F. to 415.degree. F.
Early experiments in baking bread involved the serial application of microwaves and conventional heat, i.e., first applying microwaves to obtain the oven spring and setting the structure and then finish baking with conventional heat to obtain the much needed crust color. While some degree of success was seen, particularly the 915 MHz frequency, the finished loaves left much to be desired since they tended to collapse, had poor rubbery interior structures and very rough unusual crust characteristics. The initial work was done using "Pyrex" and "Corning Ware" type baking dishes, however, here was excessive collapse due to poor sidewall formation. A significant improvement was made using paparboard containers, however, according to the invention a quantum jump in improvement of product quality began with the use of metal baking pans. This is contrary to all accepted practice in microwave processing. Two publications: K. Lorenz et al, "Baking with Microwave Energy", Food Technology, December, 1973: and T. H. Collins, "Exploring the Baking of Bread with Microwaves", FMBRA Bulletin (3), P. 175, 1970; both describe the use of metal baking pans for bread baking in a microwave field as an impossibility since the microwaves will only penetrate the dough through the top and not bake evenly.
A very popular type of bread sold commercially as prepackaged, sliced bread is generically referred to as "firm bread". This is characterized as having more of a home-made appearance, with a square cross-section rather than a round top, a coarser more open grain than the usual foam-like white bread, and a firm texture and eating quality. It may be further characterized as being denser and of lowermoisture content than the more typical white:
Firm Bread: Density: 0.25 grams/c.c.; Moisture: 34.9% PA1 Typical White Bread: Density: 0.18 grams/c.c.; Moisture: 38.2% PA1 Firm Bread: 48 units PA1 Typical White Bread: 12 units
A further measure of difference is its "firmness" as measured analytically by an Instron Universal Tester.
Typical "Instron firmness" values are:
The manufacture of firm bread also differs from typical white bread. In order to achieve the square cross-section, the bread is baked in a covered pan rather than an open baking pan. Further, its baking time is typically 40 minutes or longer whereas typical white bread is baked in 17 to 23 minutes. It is this latter difference which is of significance to the commercial bakery since it means the ovens must run at only half their regular speed and thus production output per hour is halved. A valuable processing contribution to the baking industry would be to speed up this baking process.
Upon investigation it was found that all of the "oven-spring" (that is the expansion to full volume) occurs within the first 10-12 minutes of baking. Further, very little bake-out loss (that is moisture loss) occurs in this early time period. This being so, we investigated the method of using microwave energy to finish bake the bread after the initial baking period to achieve the volume expansion. The baking was done with the dough in metal baking pans throughout the entire baking cycle, but with the lid on during the first 10-12 minutes of conventional baking and then removing the lid and finish baking with microwaves and hot air for the last 10-12 minutes to give total baking time of 22 minutes; about a 50% savings in time.